Microstrip and stripline techniques are very popular at microwave frequencies due to their small physical size, versatility, price and ease of construction. However at millimeter wave frequencies and above, problems occur that limit the usability of such structures. The wavelengths become so small that parasitic effects brought by practical sized interconnections, such as input/output coupling and intra-layer transition vias are performance limiting, and unintended modes occur in the design. In order to reduce these effects one has to go to smaller and smaller layer thicknesses, which causes a reduction in the widths of the microstrips/striplines and exacerbating the natural metal losses of the resonators that increase with frequency. For these reasons microstrip/stripline based resonator structures can become too lossy for certain applications.
Mechanical filters offer distinct advantages at the above mentioned frequencies. The use of large metal surface resonators circumvents the electrical loss issue and coupling into and between resonators can be done through openings in the relatively large cavity resonators. However, these structures are large, hard to integrate with other components and extremely expensive
Laminate based technology has been used in the past, see U.S. Pat. Nos. 6,535,083, 6,137,383 incorporated by reference herein. See also U.S. Pat. Nos. 5,821,836, 6,362,706, 6,535,083, and 5,382,931 which disclose constructing combline bandpass filter structures incorporated by reference herein. Walls formed by plated through holes or vias define dielectric filled waveguide structures. Furthermore, plated through holes that do not go all the way through and are situated inside the structures are used as the combline resonator elements. These prior art devices are fixed frequency filters and are not tunable unless separate non-integrated elements are added which negatively affect cost and performance. Integration of separate tuning elements is possible but limitations in miniaturization and parasitics will prevent high frequency operation.
An electrically tunable filter provides great flexibility in system architectures, by being able to replace multiple fixed frequency filters and switch matrices but this can result in interconnection parasitics between filter resonators and tunable elements at these frequencies.